Transfer contexts: split send_coins_*() methods

[plafer]

To escrow/unescrow tokens, we currently

1. get the escrow address using get_escrow_address()
2. use send_coins_execute() to/from this escrow address

A better API would replace these functions with:

1. escrow_coins_{validate,execute}()
2. unescrow_coins_{validate,execute}()

These names are more descriptive, and tell the implementor if they are currently escrowing or unescrowing tokens, which in some cases is necessary information (e.g. in the Sovereign SDK).